It is important that aircraft ground taxiing phases in an airport can take place in complete safety, whatever the density of the air traffic or the visibility conditions. Aircraft landing gear often has a significant footprint and occupies an appreciable part of the width of the traffic ways. For example, the width of the landing gear of an Airbus A380 exceeds 14 meters. One of the major objectives for ensuring safety is that, during taxiing, the whole of the landing gear remains entirely on the traffic way so as to prevent the aircraft from approaching too close to obstacles in proximity to the traffic way or the landing gear from leaving the runway.
One of the tricky points of taxiing is the negotiating of turns. Specifically, as illustrated in FIG. 1, in an airplane 11, the pilot 10 is situated at a height H of several meters above the ground. Therefore, a non-visibility zone 13 is significant below the craft. Thus, in an Airbus A380, the pilot is placed 7 meters above the ground and the non-visibility zone 13 shaded in FIG. 1 extends over 25 meters. Moreover, the large distance separating the main landing gear and the nose wheel 12 does not facilitate maneuvers. In an Airbus A380, the latter distance reaches 30 meters.
On modern aircraft, to ensure the airport navigation function, there exist two types of systems for aiding traffic flow on the ground. These are:
A system for aiding navigation enabling the pilot:                on the one hand, to ascertain the position of the aircraft in the airport at any moment;        on the other hand, to indicate to him which path to follow to get from one point to another on the airport.        
Generally the useful information is presented on a so-called head-down instrument panel visualization. This information comprises:                a mobile electronic map of the airport;        the position of the aircraft on the airport maneuvering area;        the trajectory to be followed and the stopping points, the latter correspond to the taxiing authorizations provided by the ground control.        A system for aiding taxiing enabling the pilot to follow, as closely as possible, the directive trajectory coming from the control.        In the latter case, useful information is presented on a head-up collimator also called an HUD, the acronym standing for Head-Up Display. A head-up collimator conventionally comprises a source of images generating a symbology, a collimation optic and an optical combiner placed in the pilot's visual field. The collimator thus gives a virtual image at infinity of the symbology superimposed on the outside landscape.        
The symbology gives information about the trajectory to be followed and a certain number of directives. It is generated by a computer dedicated to the collimator. In the case of the system for aiding taxiing, the information is provided to the computer of the collimator by:                The main navigation system, notably in respect of the information regarding heading, ground speed and position;        The airport navigation computer, based on:                    the traffic flow directives provided by the air traffic controller, a series of traffic way segments that must be followed by the airplane during the taxiing phase, and            Information contained in a database relating to the airport platform on which the airplane is situated. There exist three categories of database giving the description of airports, called “Coarse”, “Medium” and “Fine”, defined in the document RTCA D0272/EUROCAE ED99, entitled “Users Requirements for Aerodrome Mapping Information”. For this type of application, the databases used are of the “Fine” category.                        
The capacity for global monitoring of the situation of the aircraft and the precision of the maneuvers to be performed manually depend directly on the characteristics and the ergonomics of the symbols presented to the pilot, through the HUD.
Conventionally, the symbols displayed in a head-up collimator are separated into two principal categories:                so-called compliant or 3D symbols which give a better perception of the environment of the aircraft. These symbols are particularly useful in the event of degraded visibility, for example for night traffic or in poor meteorological conditions. The virtual image provided by the collimator is superimposed on the outside elements with which they are associated, such as the horizon line, for example. In the case of taxiing aid, the symbols presented to the pilot are essentially symbols associated with the traffic way on which the aircraft is situated, the guide line painted on the traffic way is an example.        so-called non-compliant symbols also termed 2D which provide the pilot with navigation information which is, for example:                    the final destination of the route;            the next stopping point called the “limit clearance”;            the estimated time or the distance from the aircraft up to the final route point;            the changes of direction to be performed;            the ground speed of the aircraft;            the value of the magnetic heading.                        
FIG. 2 represents an example of symbols for aiding taxiing, according to the prior art, when the aircraft is tackling a turn. The symbology has been simplified and only the elements necessary for the invention have been retained. The thick lined frame of FIG. 2 and of FIG. 7 represents the angular limits of the optical combiner. This symbology comprises:                A compliant representation in 3D in which the symbols presented are superimposed on the outside elements that they represent. These symbols are:                    Traffic way axial marks 22 of rectangular shape. These rectangles are represented in perspective, their orientation and their size depending on their position with respect to the aircraft;            Traffic way lateral safety marks 21, of circular shape and which appear in the form of ellipses in FIG. 2;            The horizon line 20;                        A non-compliant representation in 2D. By way of example, a change of direction 23 is represented. This change of direction is symbolized by the text “TURN” followed by the indication of the number of meters to be covered before the next turn of the aircraft. In the example of FIG. 2, the distance remaining to be covered is 91 meters.        
This representation is appropriate as long as the craft is situated at the entry to the turn or as long as the visibility conditions are good. However, when the craft is in the middle of a turn, this representation becomes insufficient. As seen in FIG. 3a, it is entirely possible that in the middle of a turn 32, having regard to the non-visibility zone 31 represented by the shaded zone and situated in front of the craft 30, the symbology displayed no longer comprises any utilizable lateral safety mark. If, on the other hand, the visibility is reduced, the pilot is then totally devoid of information and of visual reference tags.
The patent, with filing number 05 07323, proposes an improvement of the symbology of FIG. 2, integrating notably symbols, representing studs of cylindrical form of variable height, situated outside the turns. Advantageously the height of the studs gives the pilot a representation of the situation of the aircraft in the turn.
Nevertheless, content of the pilot's restricted field in aircraft of the A380 type, or equivalent wide-body craft, and of the severe climatic conditions which may be detrimental to visibility, notably in turns, the pilot does not always have a simple criterion relating to the deviation of his actual trajectory with respect to the theoretical trajectory that he ought to follow. The symbology of the elements of the runway becomes insufficient.